Field of Invention
Single crystals having desirable optical transmission in spectral regions ranging from the ultraviolet, through the visible, and into wavelengths beyond 10 microns in the infrared are widely used in various scientific optical instruments. Good quality single crystals of calcium fluoride and barium fluoride, for example, have the ultimate possible optical characteristics for their particular compositions. In general, where the ultimate in optical performance is required, the single-crystal materials are selected for their transmission and optical scattering characteristics.
There are, however, many applications where factors such as cost, thermal or mechanical shock considerations, required physical properties such as hardness, toughness, etc., or other such considerations, dictate the use of a polycrystalline optical material. Another important consideration is that polycrystalline materials can be made much more economically in larger sizes than those available from single crystal techniques.
Various procedures for the manufacture of polycrystalline optical material are known. One method involves the hot pressing of a powder, as taught by the Carnall U.S. Pat. Nos. 3,476,690 and 3,206,297. Another involved the high temperature sintering of cold pressed materials, as taught by the St. Pierre et al U.S. Pat. No. 3,026,177.
Successful production of various optical materials has been achieved by employing these powder pressing techniques, but optical performance has been limited by certain processing shortcomings, e.g., by impurities in grain boundaries, which cause light scattering and absorption. This has been especially true as element size is increased, e.g., polycrystalline materials of six inches in diameter or more. Generally, such pressed materials show considerably poorer optical performance than the single crystal material of the same composition. Often the optical transmission is highly variable over various regions of the optical spectrum, and this is related to the particle size of the individual grains forming the material.
In addition to the above considerations, powder pressing methods require elaborate and expensive pressure generating equipment operating in the pressure range 4000 to 50,000 psi and at temperatures up to 1500.degree. C. Optical materials as large as eight inches in diameter and two inches thick have been pressed in such equipment, but various physical problems and also equipment costs limit the production of large size units. In present scientific applications durable large size optical materials are urgently required for missile, satellite, and other devices. Good optical quality laser windows free of critical energy absorptions are required in diameters of 12 inches and even much larger, and they must withstand the thermal and mechanical shocks involved in aircraft and other such usage.
The production of polycrystalline optical materials from a homogeneous melt is an alternative solution to powder pressing. The Bridgman U.S. Pat. No. 1,793,672 discloses techniques and apparatus primarily suited for the growth and purification of single crystals, but also teaches that a multicrystalline solid can be obtained by rapidly lowering a mold out of the melting zone of a crystallizing furnace. However, there are several important physical reasons why the Bridgman technique has not been found useful for formation of certain metal halide polycrystalline optical elements. Such metal halides are all ionic crystals and decrease strongly in volume in converting from liquid to solid form. They therefore readily form voids and dendritic structures within the solid body which destroy the optical usefulness of the product. Another important factor is that the ionic crystals form by nature a fairly rigid atomic structure at temperatures fairly close to their melting points. They do exhibit limited flow characteristics at temperatures considerably below the melting point, but never to the extent that would allow adequate volume compensation for the volume changes of about ten to twenty percent which occur in the freezing or solidification of same from the molten state.
It is known that certain materials readily can be formed in the polycrystalline condition without serious problems involving volume changes occurring during the freezing process. An example is polycrystalline germanium, which is available as a good large size element of reasonable optical quality. Germanium is one of the less common materials which has a higher specific volume in the solid form over that of the liquid form and for that reason a void problem as can occur with certain metal halides is not encountered. Other examples of successful polycrystalline materials are the common metals and alloys, and some of the IV-VI and V-VI compounds of the periodic system. Even where there is the usual specific volume decrease in the freezing or solidification process, factors such as non-ionic binding, high density, wide temperature ranges of low-viscosity regions considerably below the melting points, and other considerations, all or severally tend to allow the attainment of good polycrystallinity. Such favorable factors are not present for the case of the polycrystalline metal halides as has been discussed previously.